1. Field of the Invention
The present invention relates to a hydraulic circuit for driving a hydraulic cylinder of a construction machine including a working attachment, such as an excavator, for example.
2. Description of the Background Art
Background art relating to the present invention will be described, showing an excavator shown in FIG. 5 as an example. The excavator includes a lower propelling body 1, an upper slewing body 2 mounted on the lower propelling body 1 so as to be free to slew, and a working attachment 3 attached to the upper slewing body 2. The working attachment 3 includes: a boom 4 capable of being raised and lowered; an arm 5 attached to a tip end of the boom 4 rotatably about a horizontal axis extending in a right-left direction; a bucket 6 attached to a tip end of the arm 5 rotatably about the horizontal axis in a right-left direction; a boom cylinder 7 which is a hydraulic cylinder capable of being expanded and contracted so as to drive the boom 4 in a raising direction and a lowering direction; an arm cylinder 8 which is a hydraulic cylinder capable of being expanded and contracted so as to rotate the arm 5 in a pushing direction, that is, a direction away from the upper slewing body 2, and a retracting direction, that is, a direction toward the upper slewing body 2; and a bucket cylinder 9 which is a hydraulic cylinder capable of being expanded and contracted so as to rotate the bucket 6 in respective directions corresponding to an excavating operation and a dumping operation. Each hydraulic cylinder includes a bottom side fluid chamber for receiving a supply of hydraulic fluid to expand the hydraulic cylinder, and a rod side fluid chamber for receiving a supply of hydraulic fluid to contract the hydraulic cylinder.
The excavator further includes a hydraulic circuit for driving each hydraulic cylinder. The hydraulic circuit includes a hydraulic pump, a tank, and a control valve interposed between the hydraulic cylinder and the hydraulic pump and tank. The control valve has selectable positions: a position for allowing hydraulic fluid to be supplied to the bottom side fluid chamber of the hydraulic cylinder and allowing hydraulic fluid in the rod side fluid chamber to be discharged; and a position for allowing hydraulic fluid to be supplied to the rod side fluid chamber and allowing hydraulic fluid in the bottom side fluid chamber to be discharged, thereby enabling the expansion/contraction operations of the hydraulic cylinder to be controlled.
Each hydraulic cylinder has a difference between a sectional area of the bottom side fluid chamber and a sectional area of the rod side fluid chamber, the difference corresponding to a sectional area of a rod of the hydraulic cylinder. This sectional area difference makes a flow rate of the hydraulic fluid returning to the tank, namely return fluid, from the bottom side fluid chamber during a contraction operation of the hydraulic cylinder be greater than that of the hydraulic fluid supplied to the rod side fluid chamber, thereby involving a problem of increasing pressure loss in a return side fluid passage.
In response to this problem, a first conventional technique described in Japanese Patent Application Publication No. 2002-339904 uses a quick return circuit branched off from a bottom side line of the hydraulic cylinder and communicated with the tank bypassing the control valve. The quick return circuit returns a part of the return fluid discharged from the bottom side fluid chamber during the contraction operation of the hydraulic cylinder directly to the tank, thereby reducing the pressure loss in the return side fluid passage.
The quick return circuit, however, reduces a flow rate of the hydraulic fluid returning to the tank via the control valve, thus increasing a possibility of cavitation in a supply side fluid passage particularly when the hydraulic cylinder is operated in an identical direction to a direction in which gravity acts on the attachment itself. Specifically, for example, when the arm cylinder 8 is driven in the arm pushing direction, or in other words the direction for contracting the arm cylinder 8, from such an attitude that the arm 5 is enfolded, as shown in FIG. 5, or in other words an attitude in which the weight of the arm 5 and the bucket 6 acts on the arm cylinder 8 in a direction for contracting the arm cylinder, a back pressure of the arm cylinder 8 is reduced to make a rod side pressure, that is, a supply side pressure, be negative, thereby generating a possibility of cavitation.
Meanwhile, a second conventional technique described in Japanese Patent Application Publication No. 2004-92247 uses a recycling circuit interconnecting a rod side line and the bottom side line for the hydraulic cylinder to prevent cavitation due to a reduction in the pressure in the supply side fluid passage from occurring. The recycling circuit suppresses the reduction in the supply side pressure by recycling a part of the discharge side fluid to return to the supply side bypassing the control valve. However, it is difficult to apply the recycling circuit according to the second conventional technique directly to the first conventional technique for the purpose of prevention of cavitation in the first conventional technique. If the recycling circuit were added to the first conventional technique to return a part of the return fluid from the bottom side fluid passage to the rod side fluid passage, or in other words if the recycling circuit were simply added to a hydraulic circuit including the aforethe quick return circuit, the fluid supplied to the bottom side fluid chamber for moving the hydraulic cylinder in an expansion direction could return to the tank through the quick return circuit or could be flowed into the rod side through the recycling circuit. This hinders the hydraulic circuit from functioning as a circuit for actually driving a hydraulic cylinder.